THE CIRCULATION OF THE BLOOD AND LYMPH 97 



more of its segments, the dicrotic wave, as will be readily understood 

 from the manner in which it is produced, either disappears altogether 

 or is markedly enfeebled. But apart from any anatomical lesion or 

 functional defect in the aortic valves, the prominence of the wave 

 varies with a great number of circumstances, some of which are in a 

 measure understood, while others remain obscure. It varies in 

 particular with the abruptness of discharge of the ventricle and the 

 extensibility of the arteries. The conditions are usually favourable 

 when the arterial pressure is low, for the blood then passes quickly 

 from the ventricle into the arteries, which, already only moderately 

 tense, are easily dilated by the primary wave, then sharply collapse, 

 and are again abruptly distended when the dicrotic wave arrives. 

 And, in fact, an exaggeration of the dicrotic wavelet may be artifi- 

 cially produced by nitrite of amyl (Fig. 89, p. 193), a drug which 

 lessens the blood-pressure by dilating the small arteries. Muscular 

 exercise (Fig. 88, p. 193), running or bicycling, for instance, has a 

 similar effect on the sphygmogram, although the explanation can 

 scarcely be the same, since the blood-pressure mounts rapidly when 

 moderate exercise begins and only gradually falls during its con- 

 tinuance, with an abrupt decline to normal or below it on cessation 

 of work (Bowen). The increase in the pulse-rate may have some- 

 thing to do in this case with the exaggeration of the dicrotism, which 

 is very frequently, although by no means invariably, associated with 

 a rapidly-beating heart, and therefore is often seen in fever. On 

 the other hand, in certain diseases associated with a high arterial 

 pressure the dicrotic elevation almost disappears. Atheromatous 

 arteries, being very inextensible, do not allow a dicrotic pulse. 



Since the pulse represents a periodical increase and diminution in 

 the amount of distension of an artery at any point, the line joining 

 all the minima of the pulse-curve will vary in its height above the 

 base-line, or line of no pressure, according to the amount of per- 

 manent distension, i.e., permanent blood-pressure, which the heart 

 in any given circumstances is able to maintain. Any circumstance 

 that tends to lessen the permanent distension will cause a fall of the 

 line of minima, and any circumstance tending to increase the disten- 

 sion will cause that line to rise. If, for example, the arm be raised 

 while a pulse-tracing is being taken from the wrist, the line of 

 minima falls because the permanent pressure in the radial artery is 

 diminished. 



The form of the pulse-curve varies in the different arteries, 

 and therefore in making comparisons the same artery should be 

 used. When the wave of blood only enters an artery slowly, 

 the ascending part of the curve will be oblique. This is normally 

 the case in a pulse-curve of a distant artery, such as the posterior 

 tibial. The height of the wave is also less than in an artery 

 nearer the heart, such as the carotid, or even the axillary, where 

 the primary elevation is relatively abrupt (Fig. 90, p. 193). 



Anacrotic Pulse. As a rule, the ascent of the tracing is 

 unbroken by secondary waves, but in certain circumstances 

 these may appear on it. This condition, which, when well 

 marked at any rate, may be considered pathological, is called 

 anacrotism (Fig. 32). It is seen when the discharge of the left 

 ventricle into the aorta is slow and difficult e.g., in old people 



7 



